The present invention relates to electronic controllers for motors adapted to drive varying or less than full mechanical loads, and more particularly, to control circuits for automatically reducing the power applied to a less-than-fully-loaded AC induction motor, such reduced power application reducing losses caused by reactive current to improve the power factor of the induction motor.
U.S. Pat. No. 4,052,648 to Nola discloses an AC induction motor control circuit of the subject type which utilizes a thyristor means such as a Triac switch (TRIAC is a trademark of The General Electric Company of Syracuse, N.Y.) in series with an induction motor to lessen the time of supply voltage application to the motor, on a half-cycle basis, the time of supply voltage application being inversely proportional to the power factor (greater current lag with lessening mechanical load) which is sensed by load voltage and load current sampling. In effect, Nola continuously senses the phase angle between the load voltage and load current, and then uses a phase angle-related signal to continuously adjust the firing point of the Triac switch relative to the zero crossing point of the line voltage. For a sensed increasing phase angle (decreasing power factor) between load voltage and load current, Nola shifts the Triac switch firing point away from the line voltage zero crossing point to apply a smaller portion of each half-cycle of the line voltage, which inherently decreases the phase angle (increasing power factor) and reduces the heat loss (I.sup.2 R) caused by the reactive current.
While Nola recognizes the energy-saving advantages of duty cycle controlling an induction motor as a function of its load with a series-inserted, phase-triggered Triac switch, his phase angle measuring requirement and the resultant circuitry are undesirably complex and costly as compared to the relative simplicity and low cost of a small, single phase induction motor which exhibits the greatest need for reliable power factor regulation.
U.S. application Ser. No. 042,608, filed May 25, 1979, by the inventor in the present application discloses an electronic controller which senses load current only in providing effective power factor control of an induction motor. While this current sensing only controller represents a substantial improvement over the earlier-discussed Nola device, it still requires a considerable number of components, resulting in costs which detract from its advantages in some applications.